1. Field of Invention
This invention relates to lenses for mass spectrometers, specifically to such lenses which are used at the entrances and exits of multi-pole assemblies with oscillatory and direct current potentials, such as quadruple mass spectrometers, multi-pole ion guides and collision cells, etc.
2. Prior Art
Quadrupole mass spectrometers commonly use an entrance lens in front of the quadrupole with a can or chamber encasing the quadrupole assembly to contain the oscillating fields of the quadrupole assembly inside of a metal can to prevent these fields from reaching out into the ion source region; detrimentally influencing the trajectories of ions. These fields at the entrance (and exit) of quadrupoles are commonly referred to as “fringe fields” and are composed of axially projected fields.
Originally entrance lenses where a flat plate with an aperture, referred to as a “shime or a diaphragm” (Steffen, 1965; Wollnick et al. 1965). However, this just prevented the fringe fields from entering the ion source. But as ions passed through the aperture, and were directed towards the quadrupole assembly, they were subjected to these fringe fields which lead to dispersing or defocusing the ion beam. Thereby, causing some ions to be lost (or rejected) and not enter the quadrupole assembly.
Thereafter, inventors disclosed several types of entrance lenses to introduce ions into a quadrupole assembly in such a way as to reduce these disperseive fields. U.S. Pat. Nos. 3,129,327 (1964), 3,371,204 (1968), 3,555,271 (1971), 3,783,279 (1974) all to Brubaker or Brubaker et al. disclosed a quadrupole assembly commonly referred to as a “pre-quad” disposed between the quadrupole mass spectrometer and the entrance lens. This pre-quad was relatively short compared to the quadrupole mass spectrometer and only powered with the RF electrical component or a derivative potential of the quadrupole mass spectrometer. Thereby, controlling the electrodes of the pre-quad to produce a decrease in the ratio of the static (DC, direct current) to the peak alternating potential (AC, alternating current)—delaying the onset of the DC component, with the ratio of DC to AC potential substantially zero at the inlet end of the quadrupole mass spectrometer.
Several types of entrance lens that are conical or tubular (snouts) shaped have been disclosed—for example U.S. Pat. No. 3,560,734 to Barnett et al. (1971), U.S. Pat. Nos. 3,867,632 (1975), 3,936,634 (1976), 3,937,954 (1976), and 4,013,887 (1977) all to Fite, and U.S. Pat. No. 6,153,880 to Russ IV et al. (2000). Barnett et al. disclosed an entrance lens comprised of two flat plates with conical (or tubular) snouts whose apexes are positioned inside the entrance of the quadrupole assembly an equal distance between each rod penetrating the fringe fields present at the entrance. When ions are accelerated through the lens into the central axis of the quadrupole mass analyzer they are shielded from these fringe fields. As the ions near the exit of the snout inside of or at the entrance to the quadrupole assembly they experience the defocusing effect of the fringe fields that are substantially reduced but still present.
Fite disclosed in a series of patents an entrance lens comprised of a flat metal plate with a dielectric tube (or snout) whose exit also is positioned inside the entrance region of the quadrupole assembly. The dielectric tube permitted the oscillatory fields from the quadrupole mass spectrometer to penetrate the tube thereby focusing the ions, as shown with the pre-quad by Brubaker, but block the defocusing direct current fields. The ions exit the tube in a similar fashion to the lens described above by Barentt et al. where the defocusing effect of the fringe fields are substantially reduced but still present
Russ IV et al. disclosed an entrance lens that is conical in shape that penetrates slightly into the central axis of the quadrupole assembly where the voltage supply for the lens is phase coherent with the voltage applied to the mass filter allowing more ions to be transmitted through the lens and into the mass filter. But nevertheless all the lenses at the entrances of multi-pole assemblies heretofore known suffer from a number of disadvantages:
(a) Entrance lens such as “shims or diaphragms” do completely block these defocusing fringe fields upstream of the entrance lens but ions upon entering and passing through the aperture of the lens, and before entering the multi-pole assembly, experience defocusing fringe fields (both axially alternating and direct current fields) which lead to the lost of ions as they traversed the distance from the lens to the multi-pole assembly—before they enter the multi-pole assembly.
(b) The use of pre-quads eliminates the defocusing DC fringe fields at the entrance of the pre-quad (delays the DC fields) but the axially RF defocusing fields remain.
(c) Lens with a metal snout or tube only shield the ions while the ions are inside the snout, but as they near the exit of the tube they experience these fringe fields in an increasing manner and some of the ions are lost, impacting into the inside walls of the tube and onto the rods of the quadrupole analyzer after they exit.
(d) Lens with a metal plate and a dielectric snout offer some improvement, but as the ions pass from the flat metal plate into the dielectric tube they experience these axial oscillatory fields and are potentially lost to the inside walls of the dielectric tube. In addition, as charged and neutral material accumulate on the inside of the tube the dielectric nature of the tube changes requiring constant adjustment of the potential of the tube; and cleaning.
(e) Combining pre-quads with either a lens with a metal or dielectric snout doses not eliminate the axial oscillating defocusing fields as the ions exit the tubes.
(f) If the radio frequency phase of the quadrupole assembly is applied to the entrance lens, ions experience defocusing fields upstream of the entrance lens and are possibly lost before passing through the entrance lens and into the multi-pole assembly.
3. Objects and Advantages
Accordingly several objects and advantages of the present invention are:
(a) to provide entrance and exit lenses for a quadrupole mass analyzer which can cancel or neutralize the defocusing fringe fields present at the inlet and outlet of quadrupole mass analyzers;
(b) to provide an entrance lens which will allow a larger percentage of ions from an ion source to pass through the entrance lens and into the central axis of the quadrupole mass analyzer uninhibited;
(c) to provide an entrance and exit lenses for quadrupole mass analyzers which can replace existing lens;
(d) to provide lens whose production allows for the individual parts to easily removed, disassembled, cleaned, and reassembled;
(e) to provide an entrance and exit lens for a multi-pole collision cell which can restrict the flow of gas out of the collision cell into the surrounding vacuum chamber; and
(f) to provide an exit lens from an high pressure multi-pole ion guide which can restrict the flow of gas out of the ion guide into the surrounding vacuum chamber.
Further objects and advantages are to provide a lens which can be easily installed and inexpensive to manufacture; which can be mass produced; can be comprised of metal, such as, stainless steel and insulating material, such as, Teflon or Vespel; which can be used with multi-pole assemblies such as a quadrupole mass analyzers, hexapole, octopole or quadrupole ion guides or collision cells; use with multi-plate ion guides or collision cells; as an exit lens for a multi-pole assembly; can replace entrance lens in electron ionization ion sources to quadrupole mass analyzers; and can be easily retrofitted to existing assemblies or instrumentation. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.